Jaspar entered politics in the Catholic Party, was appointed minister for economic affairs in 1918, and helped to organize reconstruction of Belgian industries after World War I. As foreign minister (1920–24), he facilitated Belgium's entry into the League of Nations and also the country's representation at the Permanent Court of International Justice at The Hague. He helped create the customs union of Belgium and Luxembourg in 1921, and in 1924 he took part in the Dawes Plan negotiations, which enabled Germany to resume reparations payments to the Allies.

Jaspar became prime minister of a coalition government in 1926 and implemented a series of financial policies designed by his finance minister, Émile Francqui, to remedy the economic crisis; these measures included devaluation of the currency, creation of new taxes, conversion of the public debt, nationalization of the railroads, and financing of public works. These measures revitalized the Belgian economy, although it declined again after the onset of the Great Depression in 1929. Jaspar served as finance minister (1932–34) and foreign affairs minister (1934). He was asked to form a new government in 1939 but died soon thereafter.